The present invention relates to a device for energizing a voltage-source converter, the converter being intended for connection on its ac-voltage side to a three-phase electric power network via a transformer and on its dc-voltage side to capacitor equipment, the transformer having resistor equipment for limiting the current through the converter when connecting the transformer to the power network.
Capacitors or capacitor equipment comprising a plurality of capacitors are used in electrical power engineering among other things for achieving, on a dc voltage network, a stable dc voltage with a low harmonic content. One example of such an application is in installations for transmission of high-voltage direct current, where so-called voltage-source converters are used for converting an ac voltage to a dc voltage, and vice versa. In these applications, the converters are connected, on their dc-voltage side, to a more or less extended and branched dc-voltage network. Another example of an application is in installations for compensation of reactive power, where such a voltage-source converter is connected, on its ac-voltage side, to an electric power network and is connected, on its dc-voltage side, to capacitor equipment only. In such installations, the converter is usually connected to the ac-voltage network via a converter station, which normally has a Y-connected secondary winding.
When installations of the above-mentioned kind are not in operation, the capacitor equipment is discharged and when resuming the operation it has to be charged again to provide a controllable function of the converters. Converters of the above-mentioned kind comprise, in a known manner, semiconductor valves with branches of series-connected semiconductor elements capable of being turned on and turned off, for example power transistors of a so-called IGBT type, and diodes antiparallel-connected thereto. The charging of the capacitor equipment to the desired voltage level, that is, the energizing of the converter, may take place by rectifying the voltage of the ac-voltage network via these diodes. Connecting full line voltage directly to the converter, however, initially results in a heavy surge through the converter in that the capacitor equipment at this stage does not exhibit any counter voltage. Admittedly, the current decays concurrently with the voltage across the capacitor equipment growing, but the loss power developed in the diodes may still be sufficient to damage the diodes. Other components in the installation are, of course, also subjected to the high current stress.
To solve the above problem, various conventional solutions may be resorted to. Thus, for example, an auxiliary transformer with a ratio of transformation different from that of the converter transformer may be connected to the dc-voltage circuit, either via a separate rectifier or via the converter, whereby a certain limitation of the initial current is obtained. However, this means that extra components have to be included in the installation, which, of course, complicates the installation and renders it more expensive. Another known method of limiting the current is to temporarily connect resistors, between the transformer and the converter, in series with each one of the phase windings of the transformer, by means of switching elements in the form of circuit breakers, which resistors thereafter, in dependence on some criterion, for example after a certain time, are bypassed. Also in this case, extra components are required, which complicates the installation and renders it more expensive.
The object of the invention is to achieve an improved device for energizing a voltage-source converter which, on its dc-voltage side, is connected to capacitor equipment, and which, particularly for the applications mentioned in the introduction, provides a simple and appropriate and thus cost-effective design of the installation.
This object is achieved by using a small number of components for solving the task and particularly by utilizing components already designed for this purpose.
In a first embodiment of the invention, in which the transformer on its secondary side has a first, a second and a third phase winding, each one with a first and a second winding terminal, respective first winding terminals for interconnection to a common neutral point, and respective second winding terminals for connection to the converter, the resistor equipment comprises a first resistor, connected to the first winding terminal of the second phase winding, and the device comprises switching equipment intended, in an initial position, to block current through the phase windings; in a transition position, to form a current path which comprises at least the first and the second phase winding and, in series with these, the first resistor, which current path, when the converter is connected to the transformer, closes through the converter and the capacitor equipment; and, in an operating position, to interconnect all the first winding terminals, to form the common neutral point.
In a second embodiment of the invention, the resistor equipment comprises, in addition thereto, a second resistor, connected to the first winding terminal at one of the first and the third phase winding, and the switching equipment comprises a changeover switch with at least two mutually synchronized three-position contact groups and with a common motor-operated drive mechanism for operating the contact groups, to form, in the transition position, current paths which comprise the above-mentioned resistors and, in series with each one of these, the phase winding to which the respective resistor is connected, which current paths, when the converter is connected to the transformer, close through the converter and the capacitor equipment, and whereby, in dependence on a start order, the drive mechanism automatically, by means of a continuous rotational movement, brings the changeover switch to move from the initial position to the operating position while passing the transition position, in which the changeover switch is situated at a time determined by the rotational movement of the drive mechanism.
One of the advantages achieved by the invention, especially compared with conventional uses of resistors and circuit breakers for current limitation which require three resistors and three circuit breakers, is that the number of components is reduced. Thus, the first embodiment of the invention requires only one resistor and three switching members, whereas in the second embodiment the necessary switching operations can be carried out with only a slightly modified standard component which, per se, also comprises an appropriate control of the switching sequence.